Potassium Hydroxide
Also known as Caustic potash, Potassium hydrate, Lye (potassium), KOH
“CIR Expert Panel says: safe as used in cosmetics.”
Potassium Hydroxide (caustic potash; CAS 1310-58-3; KOH) is a strong inorganic base used as a pH adjuster in cosmetic formulations. The Cosmetic Ingredient Review (CIR) Expert Panel assessed Potassium Hydroxide together with other inorganic hydroxides (sodium hydroxide, calcium hydroxide, magnesium hydroxide, lithium hydroxide) and concluded these ingredients are safe in hair straighteners and depilatories under conditions of recommended use — with the qualification that users should minimize skin contact — and safe for all other present practices of use and concentration when formulated to be non-irritating (Final Report December 2015). A 2021 amended re-review (Burnett et al., Int J Toxicol, PMID 34514896) reaffirmed these conclusions. Potassium Hydroxide and Sodium Hydroxide share EU Cosmetics Regulation Annex III Entry 15a, which restricts maximum concentrations to 5% in nail cuticle solvents, 2% (general) and 4.5% (professional) in hair straighteners, pH below 12.7 in depilatories, and pH below 11 in other uses. KOH is functionally equivalent to NaOH as a strong base; at trace pH-adjuster concentrations in leave-on products it is fully neutralized and not caustic.
Strong-base pH adjuster in cosmetic formulations — raises pH, neutralizes acidic components, and enables carbomer gel formation and fatty-acid saponification (producing potassium soaps)
Functionally interchangeable with sodium hydroxide for pH adjustment; KOH is sometimes preferred where a potassium counterion is desired (e.g., soft soap manufacture)
CIR Expert Panel concluded safe as used in cosmetics with qualified restrictions on high-concentration applications (Final Report December 2015; amended re-review Burnett et al. 2021)
- · Irritating and corrosive in animal studies at 2% or greater (rabbit skin) and 10% (guinea pig); these findings underlie the SQ qualified-safe finding rather than outright safe
Caustic/corrosive at high concentrations — in hair straighteners and depilatories direct skin contact can cause chemical burns; CIR qualifies these uses as safe only under conditions of recommended use with the caveat that users should minimize skin contact
EU Annex III Entry 15a imposes hard concentration caps by product type: 5% in nail cuticle solvents; 2% (general) or 4.5% (professional) in hair straighteners; pH below 12.7 in depilatories; pH below 11 in all other uses
CIR Quick Reference Table (12/2017, revised 07/2018) — POTASSIUM HYDROXIDE row: Finding=SQ, Citation=Final report 12/2015 available from CIR
“Potassium Hydroxide | SQ | safe in hair straighteners and depilatories under conditions of recommended use; users should minimize skin contact; all other uses safe when formulated to be non-irritating | Final report 12/2015 available from CIR”— QRT-122017revised072018.pdf, p. 111